A typical analyzer of fixed indoor type for evaluating the potential of petroleum source rock is adapted to analyze hydrocarbons and CO.sub.2 evolved by thermal cracking of kerogen (insoluble high molecular organic substances in sediments are called "kerogen") in source rock; thus, it operates on the principle of rapidly heating about 100 mg of source rock sample in He stream from room temperature to 250.degree. C., detecting the known hydrocarbons evaporating from the sample as an S1 peak by a hydrogen flame ionization detector (FID), heating the sample to about 550.degree. C. at a heating rate of 25.degree. C./min and detecting also by FID the hydrocarbons evolved by thermal cracking as an S2 peak. On the other hand, some of the thermally cracked gases flow as a branch stream to a CO.sub.2 trap, where CO.sub.2 being evolved at temperatures between 250.degree. C. and 390.degree. C. is arrested and then it is detected as a peak S3 by a gas chromatograph connected to a heat conductivity detector. In addition it has been generally accepted that since CO.sub.2 is evolved by decomposition of inorganic carbonates at temperatures above 390.degree. C., it suffices to detect the amount of CO.sub.2 being evolved from organic carbonates as far as CO.sub.2 in kerogen analysis is concerned. The type of kerogen can be determined from S1, S2, S3 and the peak temperature (Tmax) at which S2 is obtained in such analysis.
However, the apparatus described above is designed to be used in laboratories, requiring complicated control operation and using He gas requiring a steel gas cylinder and H.sub.2 gas requiring a steel gas cylinder and liable to explode; thus, it is not suitable for use in the field. Therefore, at present, all of the samples taken are transferred to a location where such apparatus is installed (sometimes they are sent abroad) while it is still unknown whether or not they are worthy of analysis. This should be called a very wasteful analytical method from the standpoint of efficiency of analysis.
There is a recently developed portable measuring apparatus known as "source rock analyzer". This portable apparatus operates on the principle of using two large and small sieves (4.0 mm-5 mesh and 3.5 mm-6 mesh) for sorting rock sample particles taken in chip form, placing them in a thermal cracking furnace in the form of a small-sized crucible, instantaneously heating them in an air atmosphere to about 700.degree. C. to evolve hydrocarbons, detecting said hydrocarbons by a contact combustion type gas sensor, and evaluating the amounts of organic substances contained in the sample in two-class evaluation (lean/rich decision).
This portable apparatus, featured as a small-sized thermal cracking furnace, has merits that (1) the analyzing time is shortened and that (2) the capacity of shearer is low and hence the battery is small-sized, resulting in the entire apparatus being small in size and light in weight, and it is suitable for use in the field. In a performance aspect, it has the following demerits.
(1) The rapid heating of a sample in an air atmosphere causes some of the hydrocarbons evolved to be burnt, leading to errors in measurement. At the same time, the CO.sub.2 produced by combustion join the CO.sub.2 produced by thermal cracking, making it impossible to identify these gases;
(2) Even if chip-like sample particles are sifted, their variations in weight are still great, affecting the measurement of hydrocarbons; and
(3) The thermal cracking furnace is in the form of a crucible, with a heater wound naked on the inner surface, so that there is a difference in the attained temperature of the sample particles between the time the sample particles in the crucible are contacted with, the heater and the time they are not contacted, so that the amount of hydrocarbons produced differs from place to place.